X-linked Retinitis Pigmentosa is one of the most sever e of the hered- itary causes of blindness in man, yet its biochemical basis remains un- known. Recent linkage data indicate the existence of two distinct loci, both regionalized to the proximal short arm of the X chromosome: one at Xp (RP3) and the other more proximal linked to DXS7 (RP2). This research proposal aims to clone and characterize the gene for RP3 and define further the chromosomal localization of the RP2 locus. To isolate the RP3 gene, genomic clones recently obtained from the region between the CGD locus and JBB will be analyzed further in order to identify conserved sequences indicative of exon containing sequences and to detect either microdeletions or rearrangements in DNA panels of unrelated males with XRP by either Southern blot analysis or pulse field gel electrophoresis. Additional clones that map close to or within the RP3 locus will be iso- lated by bidirectional chromosomal walking from the nearest flanking markers in phage libraries and by analysis of already obtained Yeast Artificial Chromosome (YAC) clones that contain sequences from this region. Clones that detect conserved sequences will be used to isolate candidate RP3 cDNAs which will be further characterized by Northern analysis and sequencing. Once a candidate sequence had been found, future efforts will be targeted towards identifying mutations at the RP3 locus, elucidating the structural organization of the gene and developing reagents to study the cellular and biochemical properties of the RP3 protein. To localize more precisely the RP2 locus a detailed long range restriction map that will link up regions between DXS7 and DXS255 and identify HTF islands will be developed with the aim of identifying the restriction fragment that contains the RP2 gene.